1. Field of the Invention
The present invention relates to a resin composition comprising mainly polylactic acid and aliphatic polyester carbonate, as well as a molded article thereof.
The resin composition comprising mainly polylactic acid and aliphatic polyester carbonate according to the present invention is superior in flowablity and moldability and suitable for obtaining injection-molded articles, extrusion-molded articles, vacuum-formed articles, blow-molded article, fibers, multifilaments, monofilaments, ropes, nets, woven fabrics, knittings, nonwoven fabrics, films, sheets, laminates, vessels, foams, various parts and other molded articles. The resulting molded articles have adequate mechanical strength and thermostability and simultaneously they can be microbially decomposed easily in soil, active sludge and composts.
Accordingly, the present invention can be applied to a wide variety of uses where recycling is difficult in the fields of packaging materials, agriculture, fishery, foods etc.
For example, in the field of packaging materials, the molded article of the invention can be used as a film for packaging and can also be heat-sealed. Further, it can be utilized as a multi-film for covering the surface of soil to keep the temperature of the soil, as a pot and rope for garden plants, or as a coating material for fertilizers in the field of agriculture, or can be utilized as a fishing line, a casting net in the filed of fishery, or as a sanitary material such as medical materials and sanitary items in the medical field.
Further, the present invention relates to a biodegradable injection-molded article having improvements in mechanical characteristics and impact resistance as well as superior disposal properties.
2. Description of the Related Art
In recent years, there arises a need for development of high-molecular materials decomposable in the natural environment for environmental problems in the global scale, and particularly plastics decomposable by microorganisms are greatly expected for use as environmentally compatible materials or as a new type of functional materials. That is, molded articles consisting of biodegradable resin which can be decomposed under the natural environments are desired in recent years from the viewpoint of protection of the natural environments, and the study of naturally degradable resin such as aliphatic polyesters is actively conducted.
Heretofore, it is well-known that aliphatic polyesters are biodegradable, and typical examples of polymers obtained from aliphatic hydroxycarboxylic acids include poly-3-hydroxybutyrate (PHB) produced by microorganisms, polycaprolactone (PCL) as a synthetic polymer and polylactic acid (PLLA) produced through fermentation from L-lactic acid as the starting material.
PHB-based biopolyesters are excellent in compatibility with the environment but poor in productivity, and because of high costs, there is a limit to their use as a substitute for general-purpose plastics. PCL is obtained with high degrees of polymerization to permit it to formed into a film, but it is poor in thermostability with a melting point of 65.degree. C. or less, so its applicability is limited.
Particularly, polylactic acid is deemed most promising in respect of costs and physical properties, and it is a thermoplastic resin having transparency but suffers from problems in physical properties, such as poor elongation and flexibility as well as its low softening temperature. Further, because of the low rate of biodegradation thereof in the environment and the unstability thereof to hydrolysis, its improvements have been desired. That is, polylactic acid has a relatively high melting point of 150 to 180.degree. C. and is superior in transparency, and thus it is expected for use as a molding material. However, polylactic acid though having high strength because of its rigid molecular structure has the problem of poor impact resistance and brittleness.
Aliphatic polyesters other than polylactic acid are generally superior in flexibility and impact resistance, but their melting point is 60 to 110.degree. C. which is lower than that of polylactic acid, their glass transition temperature is room temperature or less and their crystallinity is high, so they are opaque and poor in strength.
On the one hand, aliphatic polyester carbonates obtained from aliphatic dibasic acids, aliphatic dihydroxy compounds and carbonate compounds are polymers excellent in moldability and biodegradability having physical properties similar to those of polyethylene. However, in fields where relatively high rigidity or tensile strength is required, their strength is not satisfactory. Their rigidity can be improved by use of fillers such as talc for improvement of rigidity, but there occurs the problem of reduction of flowablity etc., so it has been desired to improve rigidity without reducing flowablity.
As described above, while the existing biodegradable plastics have their respective characters, there are a number of unsatisfactory aspects, and therefore it has been desired to develop plastics with strength, thermostability, moldability and biodegradability balanced with one another.